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If there is a huge element of dark matter in the universe, wouldn't this extra gravity prevent the accelerated expansion of the universe?

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Depends on the energy density of dark matter in relation to the energy density of dark energy. –  Kyle Kanos Aug 14 at 17:34

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The acceleration of the expansion is currently observed to be happening. This observation is one of the pieces of data we use to infer the amount of dark matter. It tells us that there can't be more than a certain amount of dark matter, because that would be incompatible with the observed acceleration.

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OK, so it is a question of balancing the forces against one another to create the right conditions to explain the observations. Thanks. –  allan emery Aug 14 at 19:03

The short answer is yes, the presence of dark matter would act to counter the expansion of the universe. And in fact it does--but not enough to stop the expansion. Dark matter has gravity just like normal matter. In fact, that's pretty much the only reason we know dark mater exists at all: we can observe dark matter's gravitation effects in the rotation rates of galaxies, gravitational lensing, and things like that. Note that you shouldn't confuse dark matter with dark energy, which is presumed to be responsible for the acceleration of the expansion of the universe.

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That makes perfect sense. Perhaps if dark matter is the source of dark energy it both inhibits and causes expansion simultaneously. –  allan emery Aug 14 at 19:00
    
@allanemery be careful about jumping to hypotheses like that. The "dark" in both is a complete coincidence of naming. And moreover the ratio of dark matter to dark energy changes with the scale of the universe. –  Chris White Aug 14 at 20:29
    
Dark matter is a hypothetical component of the universe that has been imagined to explain the anomalous phenomena David Rose listed. However, one should bear in mind that those anomalies could be due to large scale inaccuracies of General relativity instead of being due to the presence of invisible gravitating objects. The same could hold for dark energy. –  Mathias Aug 20 at 0:37

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